Chai Assertions for Promises

Chai as Promised extends Chai with a fluent language for asserting facts about promises.

Instead of manually wiring up your expectations to a promise's fulfilled and rejected handlers:

doSomethingAsync().then(

function(result){

result.should.equal("foo");

done();

},

function(err){

done(err);

}

);

you can write code that expresses what you really mean:

returndoSomethingAsync().should.eventually.equal("foo");

or if you have a case where return is not preferable (e.g. style considerations) or not possible (e.g. the testing framework doesn't allow returning promises to signal asynchronous test completion), then you can use the following workaround (where done() is supplied by the test framework):

doSomethingAsync().should.eventually.equal("foo").notify(done);

Notice: either return or notify(done)must be used with promise assertions. This can be a slight departure from the existing format of assertions being used on a project or by a team. Those other assertions are likely synchronous and thus do not require special handling.

How to Use

should/expect Interface

The most powerful extension provided by Chai as Promised is the eventually property. With it, you can transform any existing Chai assertion into one that acts on a promise:

Progress Callbacks

Chai as Promised does not have any intrinsic support for testing promise progress callbacks. The properties you would want to test are probably much better suited to a library like Sinon.JS, perhaps in conjunction with Sinon–Chai:

var progressSpy =sinon.spy();

returnpromise.then(null,null, progressSpy).then(function(){

progressSpy.should.have.been.calledWith("33%");

progressSpy.should.have.been.calledWith("67%");

progressSpy.should.have.been.calledThrice;

});

Customizing Output Promises

By default, the promises returned by Chai as Promised's assertions are regular Chai assertion objects, extended with a single then method derived from the input promise. To change this behavior, for instance to output a promise with more useful sugar methods such as are found in most promise libraries, you can override chaiAsPromised.transferPromiseness. Here's an example that transfer's Q's finally and done methods:

chaiAsPromised.transferPromiseness=function(assertion,promise){

assertion.then=promise.then.bind(promise);// this is all you get by default

assertion.finally=promise.finally.bind(promise);

assertion.done=promise.done.bind(promise);

};

Transforming Arguments to the Asserters

Another advanced customization hook Chai as Promised allows is if you want to transform the arguments to the asserters, possibly asynchronously. Here is a toy example:

chaiAsPromised.transformAsserterArgs=function(args){

returnargs.map(function(x){return x +1;});

}

Promise.resolve(2).should.eventually.equal(2);// will now fail!

Promise.resolve(3).should.eventually.equal(2);// will now pass!

The transform can even be asynchronous, returning a promise for an array instead of an array directly. An example of that might be using Promise.all so that an array of promises becomes a promise for an array. If you do that, then you can compare promises against other promises using the asserters:

Compatibility

Notably, jQuery's promises were not up to spec before jQuery 3.0, and Chai as Promised will not work with them. In particular, Chai as Promised makes extensive use of the standard transformation behavior of then, which jQuery<3.0 does not support.

Working with Non-Promise–Friendly Test Runners

Some test runners (e.g. Jasmine, QUnit, or tap/tape) do not have the ability to use the returned promise to signal asynchronous test completion. If possible, I'd recommend switching to ones that do, such as Mocha, Buster, or blue-tape. But if that's not an option, Chai as Promised still has you covered. As long as your test framework takes a callback indicating when the asynchronous test run is over, Chai as Promised can adapt to that situation with its notify method, like so:

it("should be fulfilled",function(done){

promise.should.be.fulfilled.and.notify(done);

});

it("should be rejected",function(done){

otherPromise.should.be.rejected.and.notify(done);

});

In these examples, if the conditions are not met, the test runner will receive an error of the form "expected promise to be fulfilled but it was rejected with [Error: error message]", or "expected promise to be rejected but it was fulfilled."

There's another form of notify which is useful in certain situations, like doing assertions after a promise is complete. For example:

it("should change the state",function(done){

otherState.should.equal("before");

promise.should.be.fulfilled.then(function(){

otherState.should.equal("after");

}).should.notify(done);

});

Notice how .notify(done) is hanging directly off of .should, instead of appearing after a promise assertion. This indicates to Chai as Promised that it should pass fulfillment or rejection directly through to the testing framework. Thus, the above code will fail with a Chai as Promised error ("expected promise to be fulfilled…") if promise is rejected, but will fail with a simple Chai error (expected "before" to equal "after") if otherState does not change.

Multiple Promise Assertions

To perform assertions on multiple promises, use Promise.all to combine multiple Chai as Promised assertions:

This will pass any failures of the individual promise assertions up to the test framework, instead of wrapping them in an "expected promise to be fulfilled…" message as would happen if you did return Promise.all([…]).should.be.fulfilled. If you can't use return, then use .should.notify(done), similar to the previous examples.

Note when using other Chai plugins: Chai as Promised finds all currently-registered asserters and promisifies them, at the time it is installed. Thus, you should install Chai as Promised last, after any other Chai plugins, if you expect their asserters to be promisified.

In the Browser

To use Chai as Promised in environments that don't support Node.js-like CommonJS modules, you'll need to use a bundling tool like browserify. See also the note below about browser compatibility.

Karma

Browser/Node Compatibility

Chai as Promised requires Node v4+ or a browser with equivalent support for modern JavaScript syntax. If your browser doesn't support modern JavaScript syntax, you'll need to transpile it down using a tool like Babel.